In many communications systems, the information transmitted to the receiver is coupled through more than one frequency band. The information coupled in one frequency band may be of a different type or from a different source than that coupled in the other band. Voice information may be assigned to one frequency band while data is assigned to another. Or, for example, different types of data or data originating from different sources may be transmitted through different frequency bands. Or, for example, in frequency division multiplexing, a stream of information of one type or from one is source may be cyclic assigned to a particular band. In any event, in such system applications, there is a need for circuitry which inserts an information signal into and/or extracts a received information signal from a particular frequency band. We shall refer to such circuitry as a coupler.
Couplers can be provided using passive or active components. While passive components have long been used to provide the necessary coupling function, the performance objectives of certain communication system applications exceeds the capabilities of prior art couplers which utilize passive components. Couplers implemented with active devices, on the other hand, must be powered to function, and backup power capabilities must be provided in applications where it is required that the communication system continues to function during commercial power service interruptions. Providing this power backup capability adds an expense which cannot be tolerated in certain situations. It would, therefore, be desirable if the performance capabilities of a coupler implemented with passive devices could be enhanced.